<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <style type="text/css">
ul{
    list-style-type:none;
    width:100%;
    margin:0;
    padding:0;
    overflow:hidden;
    display: flex;
    justify-content: center;
    align-items: center;
}
li{
    float:left;
}
a:link,a:visited{
    display:block;
    width:120px;
    font-weight:bold;
    color:#FFFFFF;
    background-color:#bebebe;
    text-align:center;
    padding:4px;
    text-decoration:none;
    text-transform:uppercase;
}
a:hover,a:active{
    background-color:#cc0000;
}
    </style>
</head>
<body>
<ul>
<li><a href="sign_in" {% if navigation =='sign_in' %}style="background-color:#cc0000"{% endif %}>Sign in</a></li>
<li><a href="sign_up" {% if navigation =='sign_up' %}style="background-color:#cc0000"{% endif %}>Sign up</a></li>
<li><a href="">Contact</a></li>
<li><a href="">About</a></li>
</ul>
<script>
function sign_in(){
    window.location.href = "sign_in";
}

function sign_up(){
    window.location.href = "sign_up";
}
</script>
</body>
</html>